Etiology

There is strong empirical support for a genetic basis of Reading Disorder or dyslexia from behavior genetic studies. John C. DeFries and his colleagues indicate that heredity can account for as much as 60 percent of the variance in Reading Disorders or dyslexia. As for the exact mode of genetic transmission, Lon R. Cardon and his collaborators, in two behavior genetic studies, identified chromosome 6 as a possible quantitative trait locus for a predisposition to develop Reading Disorder. The possibility that transmission occurs through a subtle brain dysfunction rather than autosomal dominance has been explored by Bruce Pennington and others.

The neurophysiological basis of Reading Disorders has been explored in studies of central nervous dysfunction or faulty development of cerebral dominance. The hypothesized role of central nervous dysfunction has been difficult to verify despite observations that many children with learning disorders had a history of prenatal and perinatal complications, neurological soft signs, and electroencephalograph abnormalities. In 1925, neurologist Samuel T. Orton hypothesized that Reading Disorder or dyslexia results from failure to establish hemispheric dominance between the two halves of the brain. Research has yielded inconsistent support for Orton's hypothesis and its reformulation, the progressive lateralization hypothesis. However, autopsy findings of cellular abnormalities in the left hemispheres of dyslex-ics that were confirmed in brain imaging studies of live human subjects have reinvigorated researchers. These new directions are pursued in studies using sophisticated brain imaging technology.

Genetic and neurophysiological factors do not directly cause problems in learning the academic skills. Rather, they affect development of neuropsy-chological, information-processing, linguistic, or communication abilities, producing difficulties or deficits that lead to learning problems. The most promising finding from research on process and ability deficits concerns phonological processing—the ability to use phonological information (the phonemes or speech sounds of one's language)—in processing oral and written language. Two types of phonological processing, phonological awareness and phonological memory (encoding or retrieval), have been studied extensively. Based on correlational and experimental data, there is an emerging consensus that a deficit in phonological processing is the basis of reading disorder in a majority of cases.

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Adult Dyslexia

Adult Dyslexia

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